1. Field of the Invention
The invention relates to an intake system for a multi-cylinder engine.
2. Description of Related Art
It is known that the charging efficiency is enchanced by utilizing the inertia effect or the resonance effect of intake air within an air intake system in order to improve output performance of the engine for an automotive vehicle.
In charging by taking advantage of the inertia effect, on the one hand, when the engine is in a predetermined rotational region, or in a tuning rotational region, an intake negative pressure wave of intake air generated within the air intake port in association with opening an intake valve in an initial stage of an inspiration stroke of each cylinder is spread or travelled at sound toward the upstream side along the inside of an individual air intake passage and the negative pressure wave is reversed to positive pressure wave in a predetermined volume chamber. Then, the positive pressure wave is spread or travelled in the same pathway toward the downstream side so as to reach the same air intake port immediately before the intake valve is opened, thereby forcing the intake air into the combustion chamber by means of the positive pressure wave and enchancing the charging efficiency.
In charging by taking advantage of the reasonance effect, on the other hand, plural cylinders of the engine are grouped into plural cylinder groups so as to allow each of the grouped cylinders to have an equal inspiration stroke with each other. The individual air intake passages of the plural cylinders of each cylinder group are united into one merged air intake passage (resonance intake passage) at their upstream ends, and the merged air intake passage is in its predetermined position provided with a pressure-reversible section consisting of a colume chamber. A pressure wave of intake air travelling back and forth between the pressure-reversible section and each cylinder is caused to exhibit resonance within the merged air intake passage by coinciding a phase of a basic intake pressure wave generated in the air intake port of each cylinder of rhe cylinder group in a tuning rotation range of the engine with a phase of a reflection pressure wave reversed in the pressure-reversible section, thereby allowing the resonance to generate a pressure wave having a large amplitude due to a vibration of pressure generating mergedly within each cylinder. This resonance pressure wave forces the intake air into the combustion chamber of each cylinder, thereby enchancing the charging efficiency.
Japanese Patent laid-open (kokai) Publication No. 40,724/1985 proposes an intake system aiming at the inertia effect, in which an individual (independent) intake passage to be connected to each of plural cylinders is divided into two branch passages so as for one branch passage to have a passage length longer than the other, which are disposed parallel to each other and an upstream end of each of which is communicated with each of surge tanks (volume chambers) disposed independently and separately from each other, thereby allowing intake air to be tuned with inertia in both of the region in which the engine revolution is low and the region in which the number of engine revolution is high.
In the state in which the intake air is tuned with resonance, when the number of engine revolution outnumbers the number of tuning revolution and the former is made higher than the latter , the charging efficiency of the intake air is lowered due to the influence of a delay in spreading the pressure wave of intake air, thereby causing a remarkably large reduction in output torque of the engine. Hence, in such a fixed intake system as causing the resonance effect as will as the inertia effect, no inertia effect can be expected due to influence from or interference with resonance effect in the region of revolution tuning with inertia, when the tuning with resonance occurs in the region having a predetermined engine revolution and the tuning with inertia occurs in the region where the engine revolution is higher than the predetermined engine revolution, thus failing to avoid the reduction of the output torque.